The invention relates generally to semiconductor device and integrated circuit fabrication and, in particular, to device structures for a silicon-controlled rectifier and methods for fabricating a device structure for a silicon-controlled rectifier.
An integrated circuit may be exposed to random electrostatic discharge (ESD) events that can direct potentially large and damaging ESD currents to the sensitive devices of integrated circuits on a chip. An ESD event refers to an unpredictable electrical discharge of a positive or negative current over a short duration and during which a large amount of current is directed to the integrated circuits. An ESD event may occur during post-manufacture chip handling or after chip installation on a circuit board or other carrier. The high current may originate from a variety of sources, such as the human body, a machine component, or a carrier.
Precautions may be taken to protect the integrated circuits on the chip from ESD events. One such precaution is to incorporate an on-chip protection circuit that is designed to avert damage to the sensitive devices of the integrated circuit. If an ESD event occurs, the protection circuit triggers a protection device to enter a low-impedance state that causes the protection device to conduct the ESD current to ground and away from the sensitive devices of the integrated circuit. The protection device is clamped by the protection circuit in its low-impedance state until the ESD current is drained and the ESD voltage is discharged to an acceptable level.
A type of protection device commonly deployed in such protection circuits is a silicon-controlled rectifier (SCR). The SCR is a four-layer solid state device includes three electrodes or terminals, namely an anode, a cathode, and a gate, that are distributed among the four layers. In its quiescent state, the SCR restricts current conduction to leakage current. However, a current applied to the gate by an ESD event causes the gate-to-cathode voltage to exceed an engineered threshold, known as the trigger voltage, and initiates the conduction of a forward current between the anode and cathode. Even after the trigger voltage is removed from the gate, the SCR remains clamped to conduct the forward current so long as the conducted current from the ESD event remains above an engineered holding current. When the conducted current from the ESD event drops below the holding current, the SCR returns to its quiescent state.
Improved fabrication methods and device structures for a silicon-controlled rectifier are needed.